How to expand an object
Go in Edit mode (tab), choose the left view (3), and switch to X Ray (AltZ) to be able to select through:

Choose the Lasso Select tool (or use the shortcut):

Select the part of your mesh you want, press shift to select again:

Switch to the Move tool (or shortcut G):

Move the selection, use the gizmo to move on the Y axis or press Y to contrain on this axis: